CHAPTER III. 



SYNCHRONOUS ALTERNATORS. 



EXPERIMENT 3-A. Alternator Characteristics.* 



i Introductory. Alternating current generators are usually 

 synchronous. Any machine generator, motor or converter 

 is said to be synchronous when the current which it delivers or 

 receives has a frequency proportional to the speed of the ma- 

 chine; otherwise it is asynchronousf or non-synchronous. 



In a synchronous machine, the current or electromotive force 

 has one half -wave or alternation first positive and then nega- 

 tive for each pole passed by a given armature conductor. A 

 cycle is a complete wave of two alternations. In a synchronous 

 machine, there is, therefore, one cycle for each pair of poles 

 passed; the frequency (cycles per second) is, accordingly, equal 

 to the speed (in revolutions per second) multiplied by the num- 

 ber of pairs of poles. 



To deliver current with a frequency of 60 cycles per second 

 (7,200 alternations per minute), a bipolar alternator would have 

 to be driven at 60 revolutions per second, or at 3,600 revolutions 

 per minute; a 4-pole machine, at 1,800 revolutions per minute, 

 etc. Alternators are commonly made multipolar, and usually 

 with manyj poles, so as to avoid excessive speed. 



* The curves used to illustrate this experiment and Exp. 3-B all relate to 

 the same machine. 



t ( ia). The induction motor and the induction generator are .'syn- 

 chronous. An induction motor must run below synchronous speed, i. c., 

 there must be a certain slip, in order to produce power. An induction 

 generator, on the other hand, must be driven above synchronous speed in 

 order to generate an electromotive force. 



\ ( ib). The high speed of the steam turbine has made possible, in fact 

 has made necessary, large alternators with only few poles ; for example, a 

 bipolar 10,000 K. W. turbo-alternator, 1,500 revolutions per minute, is men- 



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